The Characterization of Cationic Pseudostationary Phases for Electrokinetic Chromatography

by Schnee, Vincent Patrick

Abstract (Summary)

Micellar electrokinetic chromatography (MEKC) and linear solvation energy relationships (LSER) have been used to characterize the solute distribution between water and self-assemblies formed from cationic surfactants containing systematic variations in structure.
One series of surfactants consisted of N-Alkyl-N-methylpyrrolidinium Ionic Liquid type headgroups. This is the first report of an ionic liquid surfactant used as the pseudostationary phase in MEKC. The solvent characteristics of these ionic liquid type surfactants did not vary in any systematic manner with increasing tail length but were found to be significantly different compared to the well-studied hexadecyl-trimethyl-ammonium; Bromide (CTAB). The new surfactants interact more strongly with polar compounds and less strongly with compounds having nonbonding or ?-electrons, and are more cohesive.
Two series of surfactants with systematic variations in head group structure were synthesized, subjected to LSER analysis, and evaluated for the separation of representative analytes. One series consists of linear alkyl substitutions on the ammonium center while the other incorporates the ammonium into alkyl ring structures of varying size. Trends were observed in the cohesivity and polarity of the linear surfactant series, both increasing with the size of the headgroup. No trends in the LSER parameters were observed in the cyclic series, but the LSER results show that the surfactants with cyclic head groups provide a significantly different solvation environment from the linear series. The performance of these two series of surfactants was evaluated for the separation of three representative sets of analytes. Representative phenolic analytes were comprised of methoxyphenols, which are of interest due to their prevalence in wood smoke. The representative amine containing solutes consisted of compounds often found in forensic urine analysis, and represent structures typical of pharmaceuticals. Six pharmaceutical corticosteroids, which are used in replacement therapy of adrenocortical insufficiency and nonspecific treatment of inflammatory and allergic conditions, were studied as representative hydrophobic analytes.
The fist example of a phosphonium surfactant as a pseudostationary phase for MEKC is introduced. Its performance and selectivity are compared to that of an analogous ammonium surfactant. The change from an ammonium to a phosphonium charge center caused differences in the cohesivity and acid/base interactions of the pseudostationary phase.
Finally, two cationic carbohydrate based surfactants were used as a MEKC pseudostationary phase for the first time. The newly characterized glucocationic phases provided differences in interactions as seen in the LSER results